Citrullus lanatus (commonly known as watermelon) is a plant native to southern Africa, believed to have originated in areas near Namibia, Botswana, and Zimbabwe (Wein, H. C. 1997). Thanks to its sweet red-fleshed fruit, watermelon has become a popular summer food throughout the world. According to Food and Agriculture Organization (FAO, 2011), world production of watermelon exceeded 104 million tons. Watermelon is a member of the cucurbitaceae family that also comprises cucumbers, squashes (including pumpkins), luffas, gourds and melons.
There are four recognized Citrullus species, C. lanatus, C. colocynthis, C. rehmii and C. ecirrhosus; all have 22 chromosomes and can be crossed with each other successfully. C. lanatus is an annual species. It has large, broad green leaves. Fruits are of medium to large size, with thick rind and solid flesh with high water content. Flesh color may be red, yellow, or white.
C. colocynthis is a perennial watermelon. It differs from C. lanatus primarily in the size of plant organs. Leaves are small with narrow lobes, and are hairy and grayish in color. Bloom is profuse in autumn, when fresh vegetative growth also occurs. Fruits are small, not exceeding 5-10 cm in diameter, with rind and spongy flesh that are always bitter.
A variety of pathogens affect the productivity of watermelon plants including viruses fungi, bacteria, nematodes, and insects (Larson et al., 2000). Watermelons are inter alia susceptible to many viruses and virus resistance is therefore of major agricultural importance (Prowidenti, 1993).
The taxonomic family Geminiviridae includes some of the most important plant viruses causing severe diseases in agricultural, ornamental and horticultural crops. Geminiviruses generally are characterized by the unique twin shape of a fused icosahedral viral particle. Geminiviruses are plant viruses which have ambisense single-stranded circular DNA genomes. The genome can either be a single component of 2500-3000 nucleotides, or two similar-sized components. They generally have an elongated, geminate capsid with two incomplete T=I icosahedra joined at the missing vertex. The capsids range from 18-20 nm in diameter with a length of about 30 nm. Viruses with bipartite genomes (begomoviruses only) have these components separated into two different particles, therefore more than one virus particle is required to infect a cell. Transmission of these viruses can be via leafhoppers (mastreviruses, curtoviruses) or via species of whitefly (begomoviruses) or via treehoppers (topocuviruses).
The geminiviruses are responsible for a significant amount of crop damage worldwide. Diseases caused by these viruses have long been recognized as a limitation to the cultivation of several important crops, including maize, cassava, bean, squash, cucurbits, and tomato. Epidemics of geminivirus diseases have arisen due to a number of factors, including the recombination of different geminiviruses co-infecting a plant, which enables novel, possibly virulent viruses to be developed. Other contributing factors include the transport of infected plant material to new locations, expansion of agriculture into new growing areas, and the expansion and migration of vectors that can spread the virus from one plant to another.
Geminiviruses comprise a large and diverse family of viruses that infect a wide range of important monocotyledonous and dicotyledonous crop species and cause significant yield losses. Geminiviruses are classified into four genera: genus Mastrevirus (e.g., Maize streak virus), genus Curtovirus (e.g., Beet curly top virus), genus Begomovirus (e.g., SLCV), and genus Topocuvirus (Tomato pseudo-curly top virus).
The genus Begomovirus contains more than 200 viral species (Fauquet et al, 2008) and belong to the taxonomic family Geminiviridae. They are plant viruses that as a group have a very wide host range. Natural hosts of begomoviruses are plant species in which the virus can replicate, cause systemic infection, and encapsidate, and from which virions are ingested and transmitted to a susceptible host by the whitefly vector (Funayama, 2001). Worldwide they are responsible for a large amount of economic damage to many important agronomic and horticultural crops such as tomatoes, beans, squash, cassava and cotton in subtropical and tropical regions of Americas, Africa and Asia. Morphologically, begomovirus particles are non-enveloped. The nucleocapsid is 38 nm long and 15-22 nm in diameter. While particles have basic isocahedral symmetry, they consist of two incomplete icosahedra—missing one vertex—joined together. There are 22 capsomeres per nucleocapsid. Begomovirus species has single stranded closed circular DNA. Most begomoviruses have a bipartite genome, meaning that the genome is segmented into two segments (referred to as DNA A and DNA B) that are packaged into separate particles. Both segments are generally required for successful symptomatic infection in a host cell, but DNA B is dependent for its replication upon DNA A, which can in some begomoviruses apparently cause infections on its own.
Watermelon Chlorotic Stunt Virus (WmCSV), a begomovirus, can cause severe losses in cucurbits, as its host range includes all the major cultivated cucurbit crops: melon (Cucumis melo), squash, cucumber (Cucumis sativus L.), pumpkin (Cucurbita maxima Duchesne), and tropical pumpkin (C. moschata Duchesne) (Jones, D. R. 2003, Lapidot, M. and Friedmann, M., 2002), however, WmCSV affects mostly watermelon plants.
The virus was first identified in Yemen in 1986 (Bedford, I. D. et al. 1994, and Jones, P. et al, 1988, and Walkey, D. G. A. et al, 1990) and later reported from Sudan (Lecoq, H. et al, 1994, Marchelo, P. W., 1996), Iran (Bananej, K. et al., 2002), Israel (Abudy, A. et al., 2010, and M. S. Ali-Shtayeh, R. M. et al, 2012), Jordan and Lebanon (Al-Musa, A. et al., 2011, Samsatly, J. et al., 2012).
Infected watermelon plants develop vein yellowing, chlorotic mottling, and severe stunting of young leaves followed by a substantial fruit yield loss (Bananej et al., 2002; Jones et al., 1988).
Current methods of preventing and controlling geminiviruses include controlling the spread of insect vectors that carry the virus, developing transgenic plants expressing the viral coat protein, and using classical breeding methods to develop plants having natural resistance to the virus. Disease resistant plants developed using classical plant breeding offer an effective, safe, and relatively less expensive method of controlling many crop diseases.
Different levels of tolerance to WmCSV were observed in Citrullus lanatus landraces from Sudan; there is however no report of resistance as such in these landraces, and no result regarding transmission of this tolerance to commercial lines.
A tolerance to Watermelon Chlorotic Stunt Virus (WmCSV) was also identified years ago in Citrullus colocynthis PI 494529 (Raaed M. Elhassan et al., 2008), but it was identified as a dominant one which was further not simply inherited and whereas minor genes could be involved. As such, its use for breeding resistant watermelon varieties may prove difficult and possibly lead to losses of levels of resistance. No data regarding successful introgression of this tolerance into watermelon varieties is reported in the literature.
Resistance to WmCSV was also identified in another related genus, namely Cucumis melo (Yousif, M, T et al, 2007). No intergenic transmission of this trait from Cucumis melo to Citrullus lanatus has however been obtained so far.
In spite of intensive work in this respect and the importance of watermelon production worldwide, currently, no C. lanatus watermelon plants resistant or tolerant to WmCSV have been obtained through introgression of the trait from a wild citrullus accession or from another related genus. This failure can be attributed to probable undesirable negative traits linked to the resistance found in other species or genus, which are hard or impossible to break, especially traits of flesh qualities (color, brix, fibers), it has thus not been possible up to now to introgress a form of WmCSV resistance while simultaneously retaining the traits of flesh qualities. Another reason for the failure to obtain resistant C. lanatus watermelon plants might come from the genetic mechanism of the previously identified source of resistance—namely a complex source with minor genes, as reported in Raaed M. Elhassan et al., 2008.
Therefore, there is an important need in the art to identify a reliable monogenic source of resistance and/or tolerance which could then be easily used to obtain resistant commercial plants of Citrullus lanatus. 
The present invention provides Citrullus lanatus watermelon plants that display resistance and/or tolerance to Watermelon Chlorotic Stunt Virus (WmCSV) and to the other geminiviruses, as well as methods that produce or identify Citrullus lanatus watermelon plants that display resistance and/or tolerance to Watermelon Chlorotic Stunt Virus (WmCSV) and other geminiviruses. The present invention also discloses molecular genetic markers, especially SNPs, linked to the recessive genetic locus conferring resistance and/or tolerance to geminiviruses.